For at least fifty years, phytoplankton and algae blooms have been a regular occurrence in summer on Lake Erie. The microscopic, floating plants generally start to flourish in June and July as the water warms and stratifies, and their numbers typically peak in August and September. But it’s not every year that a bloom leads to the shutdown of water supplies in an American or Canadian city.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured the top image, a view of an algae bloom in the west end of Lake Erie. The image of the coastal waters off of Ohio, Michigan, and southwestern Ontario was acquired at 2:50 p.m. Eastern Daylight Time (1850 Universal Time) on August 3, 2014. Algae generally gives the water a milky green color in MODIS natural-color images.

The dominant organism in the Lake Erie bloom is Microcystis spp., a type of freshwater blue-green algae that produces a toxin harmful to humans. If consumed, Microcystis can cause numbness, nausea, dizziness, and vomiting and lead to liver damage. (In rare cases, it can be deadly.) On August 2, 2014, environmental monitors for Toledo and surrounding towns in northwestern Ohio determined that public water supplies had levels of microcystin toxin that were higher than recommended by the World Health Organization (1.0 parts per billion). They warned residents not to drink or cook with tap water; boiling is not effective against the toxin. Though the bloom has continued, treatment facilities have since added extra filtering steps (including activated carbon), and public water sources were declared safe again on August 4.

Tim Davis, a harmful algae specialist with the NOAA Great Lakes Environmental Research Lab, noted that the 2014 bloom is not necessarily larger or more intense than any other recent year; it has simply had a more direct human impact because of its location in Maumee Bay. North winds recently pushed the bloom toward the northwestern Ohio shoreline and right around the water intake system for the city of Toledo. The mats of algae do not affect shipping, but they have a strong impact on water quality for consumption and recreation.

Davis was on a research boat on Lake Erie on August 4 and noted: “The water looks like someone painted the surface with green paint.” Davis and colleagues are sampling for both ongoing water quality studies and for research on the causes and solutions to the persistent blooms on the lake.

Blooms of microcystis and other species of phytoplankton occur almost every summer on Lake Erie, where chemical nutrients (mainly phosphorous and nitrogen) are plentiful due to decades of runoff from farms and cities. Research suggests that the arrival of invasive zebra mussels in the lake has changed the food web in ways that may promote certain algal species by removing competitors. And as the southernmost and shallowest of the Great Lakes, Lake Erie warms quickly and the water column tends to stagnate in layers, without much mixing to distribute heat or nutrients. Microcystis thrives in this environment, and the blooms have reached record proportions several times in recent years.